I wonder why do some airlines spend up to 12 hrs for turnround in airports? e.g., Kuwait Airways arrives in London Heathrow in the afternoon, but doesn't depart until the next morning. Is better to congest the airport rather than congesting the airspace?

In such a case, how many hours is an aircraft allowed to spend while parked on the airbridge before being moved to a remote stand? MEA spends about 3 hrs for turnround in LHR. Does its aircraft remain in the gate for all this time?

Also CX and SAA stay from urly morning to late evening at LHR. Any idea how much the average cost would be for a 747 to stand at a gate at LHR, NRT or HKG? How much would it be on a remote stand? I have travelled various times trough LHR but have never boarded an aircraft on a remote stand by bus. Does LHR does these things or are all aircraft pulled back to a gate when it is time to board?

it all depends on the willingness of the airline to pay for the parking position and if the airport needs the gate. Sometimes here at BOG we have to leave the plane at the gate because there are not parking positions at the remote location...so it changes on a daily basis
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Arrival times at the return destination are also taken into consideration. Ex. Singapore Airlines' flights from Singapore/Frankfurt to New York arrive JFK at 10am in the morning but don't depart until 10pm at night in order to make an morning arrival in Frankfurt, affording numerous connections throughout Europe.

Once i had a flight to MIA from NY i got there at aroun 1:10 Am I all ways pass the aiprot in the moring then i knew what flights and planes were there at that time. When we were taxing to the gate i saw a TWA plane that departs in the moring and a air canada plane. and a few others. I could tell which ones were leaving soon or tomorrow because they were in a stand but not conected to the gang way like the rest of the planes leaving at that time.

"It was just four of us on the flight deck, trying to do our job" (Captain Al Haynes)

Most planes take between 30 min and 90 min to turn around, and a parked airplane is a financial loser, so unless they are doing some kind of maintenance during those long stopovers, I would guess that the only reason to park an airplane for 8-10 hour stopovers would be the following hypothetical scenereo:

Dimension Airways flies a B747 from New York to Santiago, Chile once daily. Market analysis tells us that the aircraft needs to arrive in Santiago at 7:00am to accomodate business travelers. The aircraft is ready for return to New York at 8:30am, but this is inconvenient for many travelers who lose a whole business day. Therefore, if the return flight left at 8:30am, it would be a third full [of tourists] and lose money. In this hypothetical situation, there doesn't exist a market for a profitable second flight to Chile. So to avoid bringing back a 2/3 empty aircraft on the 11 hour flight back to New York (and losing the opportunity to fill up your aircraft at a different time), the airline elects to just leave it parked until 6:00pm in order to fill the seats. Sure, we're losing money but not flying the aircraft, but not as much as we would be using by operating four engines for eleven hours with minimal passenger load.

The above is only my educated guess, not a professional answer to your question.

When some airlines arrive early in the morning and travell back within 2 hours they may face the problem of arriving in the middle of the night when time-zones are involved.

Flying from west to east is a good example. Suppose you need to pass 6 timezones in a 8 hour flight. That means the aircraft will arrive 14 hours later than when it departed. So if the aircraft arrives at 9AM and departs at 11AM you will arrive at 1AM the next day. Most airports are closed at that time and not many people would like to arrive in the middle of the night. So then it would be best to wait some more hours and arrive in the early morning at the destination airport.